1. Field of the Invention
The present invention relates to a portable electronic device and a mobile communication terminal, whose surfaces do not reach a high temperature even when a user uses them while charging.
2. Description of the Prior Art
In recent years, a mobile communication terminal, an electronic databook and a PDA (Personal Digital Assistant) have been in common use as portable electronic devices. The smaller, the thinner and the lighter a portable electronic device becomes, the lower the thermal resistance. (spatial distance) between heat sources built in its body or housing (such as a speaker, an image pickup device, a storage battery being charged, or a charging circuit) and the surfaces of the housing becomes. Therefore, circuit operation tends to produce an increase in the surface temperature of a housing. In other words, in a smaller, thinner and lighter mobile communication terminal, heat produced by the built-in heat source is easily conducted to the housing, which increases the surface temperature of the housing.
Among the heat sources of a mobile communication terminal, “a charging circuit charging a storage battery” produces a lot of heat while the mobile communication terminal is in communicating state. While a mobile communication terminal is being used for communication, the terminal is often held very close to user's face. When a user uses (communicates with) the terminal while charging, the surface temperature of the housing may become so high that the user feels uncomfortable.
The Japanese Patent Application laid-open number 62-95934 discloses a “charging method” for restraining an increase in the surface temperature of the housing of a mobile communication terminal. According to the conventional charging method, the charging current is decreased while a radio device is in use to avoid a higher capacity power source and realize a smaller battery charger.
However, in the conventional charging method, the radio device (mobile communication terminal) does not have a charging circuit. In other words, the conventional charging method is not aimed at avoiding heat generation in a portable electronic device having a charging circuit.
More specifically, in the conventional charging method described above, a charger controls a switch in the radio device. In this construction, the radio device needs a dedicated charger paired with it. Consequently, if the dedicated charger is not available, as for example, when a user needs to charge a battery/the radio device at the place where he or she has gone, such problem arises as the charging current does not decrease and the heat generation in a battery cannot be avoided. With a portable electronic device typified by a mobile communication terminal, a user sometimes has to borrow a charger of someone at the place where the user travels or goes on a business trip. Therefore, it is inconvenient that the desired effect cannot be achieved without such a dedicated charger.
Further, the patent application discloses a diagram showing the structure in which both a power source and a battery feed power to a radio section, but the output voltage of the power source and the battery voltage are not the same (output voltage of the power source>battery voltage). Further, the battery voltage varies depending on how much the battery is charged. Actually, because the radio section needs a regulator (not shown) between the power source and the battery, the circuit necessarily becomes larger and more complicated. Such an increase in the size and complexity of the circuit prevents the realization of a smaller and lighter electronic device, and is thus undesirable for a portable electronic device. Moreover, because the heat produced by the regulator contributes a rise in the surface temperature of the housing, it is difficult to achieve the prevention of the temperature rise of a terminal.
As above, there has been proposed neither a portable electronic device nor a mobile communication terminal having a charging circuit and preventing the surface temperature rise of a housing while being charged.